RFC (Request For Comments) 3550 [3] describes means for transporting streams having time constraints over IP and specifies the attachment of a time reference (Timestamp) to each RTP packet.
RFC 2250[4] describes how a number of MPEG-2 transport packets can be transported within the data part of an RTP packet.
The MPEG-2 specification [1] describes how to include time references in an MPEG-2 stream and how, at the destination, the different time references are used to resynchronize the clock with that of the source. Also described is how the time references are used by the MPEG decoder to control the moments at which the elements contained in the transmitted stream are displayed.
The DVB-IP specification [5] describes a general architecture for the distribution of multimedia services over IP networks. This specification adopts RFC 2250 as the mechanism for transporting MPEG-2 streams over RTP.
The Pro-MPEG forum (Pro-MPEG Wide Area Networking group) has published a code of practice (number 3) which also adopts RFC 2250 as the multimedia stream transport mechanism. Together with DVB-IP, there is therefore an end-to-end architecture for services over IP providing interworking for equipment based on RFC 2250.
The time reference included in the MPEG-2 packets, called PCR (Program Clock Reference), is a time reference based on a clock with a precision of 27 MHz. This reference is not included in all the MPEG packets, but only in some of them. The rate of the stream is assumed constant between the packets conveying a PCR reference.
For its part, the time reference attached to each RTP packet is based on a clock with a precision of 90 kHz, therefore significantly less accurate. In the RTP case, each packet contains just one time reference. The realignment at destination of the RTP packets is therefore based on the precision of the RTP clock, or 90 kHz.